Vapor pressure control in a fractionator of an alkylation process



LQF'. MAYHUE VAPOR PRESSURE CONTROL IN A FRACTIONATOR Dec. 15, 1970 OF AN ALKYLATION PROCESS Filed May 17. 1968 C1333 BNVdOd United States Patent O 3,548,023 VAPOR PRESSURE CONTROL IN A FRACTIONA- TOR F AN ALKYLATIN PROCESS Luther F. Mayliue, Bartlesville, Okla., assignor to Phillips Petroleum Company, a corporation of Delaware Filed May 17, 1968, Ser. No. 730,066 Int. Cl. (107e 3/56; Clllg 37/06 U.S. Cl. 260-68353 6 Ciaims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION This invention relates to the conversion and separation of hydrocarbons. In accordance with one aspect, this invention relates to a fractionation process for the separation of alkylate effluents wherein the vapor pressure of the overhead removed from the fractionation is controlled by the addition of a light hydrocarbon and the vapor pressure of the bottoms alkylate removed from the fractionator is controlled by removal of a vapor side stream below the fractionation feed point. In accordance with a further aspect, this invention relates to a combination thermal cracking and alkylation process wherein the alkylate effluent is fractionated and a portion of the thermal cracking feed is used to control the vapor pressure of the overhead from the fractionator. In accordance with a further aspect, the rate of introduction of light hydrocarbon into the upper portion of the fractionation column is controlled responsive to the liquid level of an accumulation zone and the rate of withdrawal of the vapor side stream is controlled responsive to the vapor pressure of the bottoms alkylate stream.

In the manufacture of alkylate, normal butane is ordinarily included in the fresh isoparain feed as a contaminant or the butane can be contained in mixed olen feed which is obtained from normal butane cracking. The normal butane will go through the alkylation reaction substantially unchanged and appears in the alkylate product still as a contaminating component. Ordinarily, the normal butane present will be in large enough quantities to raise the vapor pressure of the alkylate above a desirable level, ordinarily 71/2 to 8 p.s.i.

In accordance with the process of the invention, in order to eliminate separate alkylate debutanizing facilities, a vapor side stream composed primarily of normal butane and heavier components is withdrawn from the fractionator following alkylation from a lower portion of the fractionation column. The normal butane vapor as well as any isopentane and heavier hydrocarbons contained in the vapor side stream are separated from the heavier side stream components. The heavier components separated will be ordinarily returned to the column as a liquid into a lower portion of the fractionation column. The normal butane and part of the isopentane are withdrawn as a vapor side draw product.

Operation of. a DIP alkylation unit with high purity ethylene feed results in an excessive amount of isobutane being lost in the depropanizer overhead product of the ice fractionation step unless a low temperature condensing unit is used on the overhead vapor stream. The refrigeration facilities increase the overall construction and operating costs of the fractionator installation significantly.

In order to obviate the necessity of a low temperature condenser and the attendant cost, in accordance with the invention the DIP alkylation system fractionator incorporates a secondary feed stream, preferably comprising propane, which is introduced near the top of the fractionation column. This propane will flash overhead from the column and leave the column as overhead product, thereby adjusting the dew point temperature of the overhead product upward while maintaining the same column pressure, and thus enable the use of a water cooled overhead vapor condenser.

Accordingly, an object of this invention is to provide an improved fractionation operation for the separation of alkylate.

A further object of this invention is to provide a combined economical process for cracking light hydrocarbons, alkylating olen eifiuent obtained from the cracking and separating alkylate eflluent. A still further object of this invention is to provide a control system for regulating the vapor pressure of the overhead from a fractionating column and vapor pressure of the bottoms product removed from the same column.

Other aspects, objects and the several advantages of this invention will be apparent to those skilled in the art upon reading the specification, the drawing and the appended claims.

SUMMARY OF THE INVENTION In accordance with the invention, in a fractionation column separating the alkylate stream, the process comprising introducing a light parafnic hydrocarbon such as propane into the top of the factionating column to lower the vapor pressure, i.e., raise the dew point, of the overhead being removed from the column.

Further in accordance with the invention, when carrying out the above fractionation a vapor side stream comprising butane and isopentane, and heavier, is removed from the fractionation column below the feed point in order to control the vapor pressure of the alkylate removed as bottoms.

In accordance with one embodiment of the invention, the overhead accumulator level controls the amount of light parafl'lnic feed, such as propane, charged to the fractionation column to lower the vapor pressure of the overhead being removed.

In accordance with another embodiment of the invention, a vapor pressure controller regulates the vapor side stream withdrawal rate responsive to the vapor pressure of the bottoms alkylate.

In accordance with a further embodiment of the invention, propane feed employed for a thermal cracking zone is used as the vapor pressure controlling agent introduced to the top of the fractionating column and the total make from the fractionator overhead is passed to a thermal cracking zone.

In accordance with a further embodiment, the efuent from the thermal cracking zone is separated to recover an ethylene stream which is contacted with isoparain, such as isobutane, in an alkylation zone to form DIP alkylate, which alkylate in turn is fractionated in accordance with the previous embodiments.

DESCRIPTION OF PREFERRED EMBODIMENTS A better understanding of the invention will be obtained by reference to the accompanying drawing which diagrammatically illustrates the fractionation system of the invention in combination with a propane thermal cracking zone and a DIP alkylation process.

Referring now to the drawing, a propane feed to be thermally cracked is introduced into the process by way of line and then passed `by way of lines 11 and 12 to thermal cracking zone 13. The cracking feed is comprised, principally, of propane but can contain, additionally ethane, isobutane and some normal butane. This feed stream is ordinarily preheated to a temperature of at least 200 F. before introduction into cracking zone 13. Within cracking zone 13 the propane containing feed is subjected to a temperature of the order of 1425 to 1525" F. to effect cracking of the propane to ethylene and other light hydrocarbon products as well as hydrogen and some heavier materials. The cracking effluent is removed from zone 13 by way of line 14 and passed to separation zone 15, which can comprise fractionation, solvent extraction, and other means to separate light hydrocarbon gases. Light gases including methane and hydrogen are removed from zone 15 by way of line 16, and propane and heavier by way of line 17. An ethylene stream is removed by way of line 18, which'is passed to alkylation zone 19.

In alkylation zone 19, fresh isobutane feed is introduced by way of lines and 21 for alkylation with ethylene-rich stream previously introduced by way of line 18. In alkylation zone 19, the isobutane and the ethylene are contacted with a suitable catalyst, such as aluminum chloride or aluminum chloride complex, or other known catalyst, under alkylation conditions to produce a diisopropyl alkylate. The alkylate, together with catalyst, is separated to produce a hydrocarbon alkylate eiuent removed from Zone 19 by way of line 22. The hydrocarbon alkylate effluent removed from zone 19 by Way of line 22 is passed to fractionation zone 23.

The alkylation conditions of temperature, pressure and ratio of reactants is well known, but these will ordinarily be: temperature, 120 F.; pressure, 400 p.s.i.a.; hydrocarbon to catalyst volume ratio, 1 to 1; isobutane to ethylene mol ratio, 10 to 1; and the catalyst is aluminum chloride complex, in a conventional diisopropyl alkylate manufacturing operation.

The hydrocarbon alkylate feed introduced into fractionation column 23 is separated into an overhead fraction removed by line 24, an isobutane side stream withdrawal by way of line 25, a vbutane and heavier vapor side stream by way of line 26, and an alkylate bottoms stream by Way of line 27 The overhead vapor stream removed by line 24 is passed through condenser 28, line 29, and then to accumulator 30. Condensate is removed from accumulator 30 by way of line 31 and passed by way of line 32 as reuX to the top of column 23. A vapor stream comprising ethane, ethylene, propane and a minor amount of heavier hydrocarbons is removed from accumulator 30 by way of line 33 and combined with the propane feed in lines 11 and 12, being passed to thermal cracking zone 13.

In accordance with the invention, a portion of the propane feed in line 10 is passed by way of lines 34 and 35 and introduced into the condensate stream 31, being removed from accumulator 30. The condensate stream together With added propane is passed as reux through line 32 to the top of column 23. The amount of propane added is suflicient to lower the vapor pressure of the overhead stream being removed by way of line 24 in order to use water cooling on the overhead from the alkylate fractionator 23. The rate of flow of propane introduced into the fractionator overhead `by way of lines 34 and 35 is controlled responsive to liquid level controller 36 and valve 37. The liquid level in accumulator 30 thus regulates the flow of propane added by way of line 34. If the propane passed through line 34 is liquid, this can be passed, then, by Way of lines and 32 to the top of the column 23. Alternatively, if the propane stream is vaporous, it is preferred that this stream be passed through valve line 38 for introduction into the overhead stream 4 24 prior to condenser 28. Again, the rate of flow is controlled responsive to the liquid level in accumulator 30. It should be realized that if the propane stream in line 34 is liquid, it need not be introduced into the top of column 23 together with the reux in line 32, but can be introduced separately near the top of the column.

Further in accordance with the invention, the butane and heavier vapor side stream removed by line 26 is passed to rectifying unit 39, wherein the vapor stream is cooled by means of indirect heat exchange unit 40, which contains a suitable heat exchange fluid. The vapor stream comprising pentane and lighter materials is removed from ectier 39 by way of line 41 as product. A condensate stream comprising pentane and heavier liquid is removed from the bottom of rectifier 39 by way of line 42, passed through pump 43, and line 44, for reintroduction into a lower portion of column 23. The rate of passage of liquid in line 44 to column 23 is controlled by liquid level controller 45 and valve 46 responsive to the liquid level in the base of rectier 39.

As indicated previously, an alkylate stream is removed from the base of column 23 by way of line 27 as product. The rate of flow of alkylate removed from the base of column 23 is controlled by liquid level controller 47, which, in turn, resets flow controller 48, which controls the flow in line 27 by regulating the position of valve 49.

The vapor pressure of the alkylate stream in line 27 is measured by Reid vapor pressure unit by passing a side stream from conduit 27 by way of line 51. The vapor pressure of the alkylate stream is in turn used to reset flow controller 52, which regulates the position of valve 53 in line 26. As the vapor pressure of the alkylate product stream varies, for example, if the alkylate vapor pressure rises, then more vapor ilow is permitted in line 26, and vice versa if the vapor pressure falls. Thus the withdrawal of the vapor side stream by way of line 26 is controlled responsive to the vapor pressure of the alkylate stream. A suitable vapor pressure unit 50 can comprise a Reid vapor pressure unit, described in Precision Scientific Development Companys bulletin Reid Monitor, No. 758. The Reid vapor pressure range normally is in the range of from 2 to 19 pounds on the alkylate, preferably 5 to 15 pounds, and more specifically 71/2 pounds.

EXAMPLE Following are the conditions obtaining for a propane crackrng system tied in with DIP alkylation and the fractionation process of the invention. The various units are indicated `by numbers, as set forth in the drawings.

mol percent Cracking Feed (10):

Ethane Propane 94. 0 isobutane 3. 0 Normal butane 2. 0 'rotar n 1o-o3 Volume, barrels/hr 15 Alkylator hydrocarbon eluent (22): Volume, barrels/hr. 1, 141

Typical mols/hr.

With Recycle to cracking zone (33) Prior invention Ethane, ethylene 26. 35

Propane Isobutane (plus).

Total, pound mois/hr Mol ratio of added propane feed to fractionator total Overhead (reflux plus make) ranges 0.005 to 0.030. This depends upon operating pressure in the condenser and the desired dew point. At 380 p.s.i.a., these give dew points of 105 F. to 150 F.

As can be readily seen from the data above, when operating in accordance with my invention, with the produced higher dew points (in the specific example, 115 E), as compared with the prior art having low dew points (in the specific example, 57 F.), I am able to use inexpensive cooling water in the indirect exchanger-condenser 28, while the prior art must necessarily employ expensive refrigeration therein.

I claim:

1. In the alkylation of isobutane with ethylene and the formation of a hydrocarbon eilluent from said alkylation comprising unreacted ethylene, unreacted isobutane, butane, pentane and alkylate, the invention steps comprising:

(a) introducting said effluent, as feed, into an intermediate portion of a fractionation zone,

(b) removing a bottoms product stream comprising liquid alkylate substantially free of butane from said fractionation zone,

(c) introducing propane into an upper portion of said fractionation zone in an amount sufficient to lower the vapor pressure of the overhead vapor stream removed from said fractionation zone,

(d) removing said overhead vapor stream comprising ethylene, propane and isobutane, which overhead stream is cooled, condensed and passed to an ac cumulation zone,

(e) removing non-condensed ethylene and propane from said accumulation zone as overhead product,

(f) returning the isobutane condensate from said accumulation zone to an upper portion of said fractionation zone as reilux,

(g) withdrawing a first vapor side stream comprising said isobutane from said fractionation zone above the feed of step (a), but below said isobutane reflux of step (f), for recycle to the alkylation Zone, and

(h) withdrawing a second vapor side stream comprising butane and heavier hydrocarbons below the feed of step (a) to control the vapor pressure of the alkylate being removed in step (b) and obviating the need for a separate de'butanizer to process an otherwise butane-contaminated alkylate.

2. A pressure according to claim 1 wherein the amount of propane introduced in step (c) is controlled responsive to the level of liquid in said accumulation Zone and is combined with the condensate passed as reflux to an upper portion of said fractionation zone.

3. A process according to claim 1 wherein said second vapor side stream in step (h) is cooled in a rectifying unit and a condensate stream comprising C5 and heavier hydrocarbons removed from said rectifying unit is returned to the fractionation zone at a point below the sec ond vapor stream withdrawal, and a third vapor side stream from the rectifying unit comprising pentane and a major portion of butane is withdrawn as product.

4. A process according to claim 1 wherein said propane removed in step (e) is a portion of a propane feed to be used in the production of ethylene in a thermal cracking zone, and further wherein said overhead product is charged as a portion of the feed to said thermal cracking zone.

5. A process according to claim 3 wherein the flow of condensate from the rectifying unit to the fractionation zone is controlled responsive to the level of liquid in the base of said rectifying unit, and the rate of flow of said second vapor side stream Withdrawal is controlled responsive to the vapor pressure of the alkylate stream withdrawn in step (b), and the rate of withdrawal of alkylate in step (b) is controlled responsive to the liquid level in the bottom of said fractionation zone.

6. A process according to claim 4 wherein an ethylene stream is separated from said thermal cracking zone and said ethylene is passed to said alkylation zone wherein ethylene is contacted with isobutane and an aluminum chloride catalyst under alkylation conditions and said hydrocarbon effluent is seperated for step (a).

References Cited UNITED STATES PATENTS 2,990,437 6/1961 Berger 260-583-43 3,004,089 10/1961 Hutto 260-6S3.48 3,431,079 3/1969 Chapman 260-683.48 3,415,899 12/1968 Van Dijk 260-683-58 PAUL M. COUGHLAN, IR., Primary Examiner G. I. CRASANAKIS, Assistant Examiner U.S. Cl. XJR. 

